On elevators with a cage velocity over 1 m/s, slippage arrestor devices are usually employed as a safety measure in case for one reason or another, the velocity of the elevator cage increases so as to become excessively high. Slippage arrestor devices engage guides in the elevator shaft, which most usually number two or four. When each guide has a slippage arrestor device of its own, the arrestor devices are synchronized by means of a separate synchronizing linkage. The slip arrestor device has a slide surface having a high coefficient of friction, which is urged against the guide when the slippage arrestor device goes into action and slows the elevator down, or stops it, with the aid of friction.
Various designs of elevator arrestor devices have been elaborated. One of the commonest types is a large-sized, U-shaped spring made of spring steel, between the ends of which the wedge enters when it engages the guide. In addition, many arrestor devices feature a separate detachment wedge, with the aid of which the arrestor device is disengaged from the guide after arresting has occurred. Disengagement is effected by raising the elevator cage.
The greatest drawbacks of prior art arrestor devices are their high price and large size. The high price is due, among other things, to the circumstance that, for instance, the springs which are used are not standardized parts. A further drawback of previously known arrestor devices is a result of the variations of force occurring in connection with the arresting action, because the value of the coefficient of friction is different at different points along the guide, depending e.g. on the surface quality of the guide, the temperature of the friction material that is used, and the velocity of the elevator cage.